#ifndef STX_OPTIONAL_HPP_INCLUDED
#define STX_OPTIONAL_HPP_INCLUDED

#ifndef STX_NAMESPACE_NAME
#define STX_NAMESPACE_NAME nonstd
#endif

// libc++ on Apple has a broken std::experimental::optional version
#if !defined(STX_NO_STD_OPTIONAL) && defined(__APPLE__)
// This header is empty on C++ but defines _LIBCPP_VERSION for us
#include <ciso646>
#if defined(_LIBCPP_VERSION) && (_LIBCPP_VERSION <= 4000)
#define STX_NO_STD_OPTIONAL
#endif // _LIBCPP_VERSION
#endif // __APPLE__

// libstdc++ and libc++'s std::experimental::optional only work in C++14 mode
#if !defined(STX_NO_STD_OPTIONAL) && defined(__GNUC__) && (__cplusplus < 201402)
#define STX_NO_STD_OPTIONAL
#endif

#if defined(__has_include) && !defined(STX_NO_STD_OPTIONAL)
#if __has_include(<optional>) && (__cplusplus > 201402)
#include <optional>
namespace STX_NAMESPACE_NAME
{
using std::bad_optional_access;
#ifndef STX_HAVE_IN_PLACE_T
using std::in_place;
using std::in_place_t;
#define STX_IN_PLACE_T 1
#endif
using std::make_optional;
using std::nullopt;
using std::nullopt_t;
using std::optional;
}
#define STX_HAVE_STD_OPTIONAL 1
#elif __has_include(<experimental/optional>)
#include <experimental/optional>
namespace STX_NAMESPACE_NAME
{
using std::experimental::bad_optional_access;
#ifndef STX_HAVE_IN_PLACE_T
using std::experimental::in_place;
using std::experimental::in_place_t;

#define STX_IN_PLACE_T 1
#endif
using std::experimental::make_optional;
using std::experimental::nullopt;
using std::experimental::nullopt_t;
using std::experimental::optional;
}
#define STX_HAVE_STD_OPTIONAL 1
#endif // __hasinclude(optional)
#endif // defined(__hasinclude)

#ifndef STX_HAVE_STD_OPTIONAL

#include <cassert>
#include <functional>
#include <initializer_list>
#include <stdexcept>
#include <string>
#include <type_traits>
#include <utility>

#define TR2_OPTIONAL_REQUIRES(...) typename std::enable_if<__VA_ARGS__::value, bool>::type = false

#if defined __GNUC__ // NOTE: GNUC is also defined for Clang
#if(__GNUC__ == 4) && (__GNUC_MINOR__ >= 8)
#define TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
#elif(__GNUC__ > 4)
#define TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
#endif
#
#if(__GNUC__ == 4) && (__GNUC_MINOR__ >= 7)
#define TR2_OPTIONAL_GCC_4_7_AND_HIGHER___
#elif(__GNUC__ > 4)
#define TR2_OPTIONAL_GCC_4_7_AND_HIGHER___
#endif
#
#if(__GNUC__ == 4) && (__GNUC_MINOR__ == 8) && (__GNUC_PATCHLEVEL__ >= 1)
#define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
#elif(__GNUC__ == 4) && (__GNUC_MINOR__ >= 9)
#define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
#elif(__GNUC__ > 4)
#define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
#endif
#endif
#
#if defined __clang_major__
#if(__clang_major__ == 3 && __clang_minor__ >= 5)
#define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
#elif(__clang_major__ > 3)
#define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
#endif
#if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
#define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
#elif(__clang_major__ == 3 && __clang_minor__ == 4 && __clang_patchlevel__ >= 2)
#define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
#endif
#endif
#
#if defined _MSC_VER
#if(_MSC_VER >= 1900)
#define TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
#endif
#endif

#if defined __clang__
#if(__clang_major__ > 2) || (__clang_major__ == 2) && (__clang_minor__ >= 9)
#define OPTIONAL_HAS_THIS_RVALUE_REFS 1
#else
#define OPTIONAL_HAS_THIS_RVALUE_REFS 0
#endif
#elif defined TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
#define OPTIONAL_HAS_THIS_RVALUE_REFS 1
#elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
#define OPTIONAL_HAS_THIS_RVALUE_REFS 1
#else
#define OPTIONAL_HAS_THIS_RVALUE_REFS 0
#endif

#if defined TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
#define OPTIONAL_HAS_CONSTEXPR_INIT_LIST 1
#define OPTIONAL_CONSTEXPR_INIT_LIST constexpr
#else
#define OPTIONAL_HAS_CONSTEXPR_INIT_LIST 0
#define OPTIONAL_CONSTEXPR_INIT_LIST
#endif

#if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_ && (defined __cplusplus) && (__cplusplus != 201103L)
#define OPTIONAL_HAS_MOVE_ACCESSORS 1
#else
#define OPTIONAL_HAS_MOVE_ACCESSORS 0
#endif

#// In C++11 constexpr implies const, so we need to make non-const members also non-constexpr
#if(defined __cplusplus) && (__cplusplus == 201103L)
#define OPTIONAL_MUTABLE_CONSTEXPR
#else
#define OPTIONAL_MUTABLE_CONSTEXPR constexpr
#endif

namespace STX_NAMESPACE_NAME
{

// BEGIN workaround for missing is_trivially_destructible
#if defined TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
// leave it: it is already there
#elif defined TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
// leave it: it is already there
#elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
// leave it: it is already there
#elif defined TR2_OPTIONAL_DISABLE_EMULATION_OF_TYPE_TRAITS
// leave it: the user doesn't want it
#else
template <typename T>
using is_trivially_destructible = std::has_trivial_destructor<T>;
#endif
// END workaround for missing is_trivially_destructible

#if(defined TR2_OPTIONAL_GCC_4_7_AND_HIGHER___)
// leave it; our metafunctions are already defined.
#elif defined TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
// leave it; our metafunctions are already defined.
#elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
// leave it: it is already there
#elif defined TR2_OPTIONAL_DISABLE_EMULATION_OF_TYPE_TRAITS
// leave it: the user doesn't want it
#else

// workaround for missing traits in GCC and CLANG
template <class T>
struct is_nothrow_move_constructible
{
	constexpr static bool value = std::is_nothrow_constructible<T, T&&>::value;
};

template <class T, class U>
struct is_assignable
{
	template <class X, class Y>
	constexpr static bool has_assign(...)
	{
		return false;
	}

	template <class X, class Y, size_t S = sizeof((std::declval<X>() = std::declval<Y>(), true))>
	// the comma operator is necessary for the cases where operator= returns void
	constexpr static bool has_assign(bool)
	{
		return true;
	}

	constexpr static bool value = has_assign<T, U>(true);
};

template <class T>
struct is_nothrow_move_assignable
{
	template <class X, bool has_any_move_assign>
	struct has_nothrow_move_assign
	{
		constexpr static bool value = false;
	};

	template <class X>
	struct has_nothrow_move_assign<X, true>
	{
		constexpr static bool value = noexcept(std::declval<X&>() = std::declval<X&&>());
	};

	constexpr static bool value = has_nothrow_move_assign<T, is_assignable<T&, T&&>::value>::value;
};
// end workaround

#endif

// 20.5.4, optional for object types
template <class T>
class optional;

// 20.5.5, optional for lvalue reference types
template <class T>
class optional<T&>;

// workaround: std utility functions aren't constexpr yet
template <class T>
inline constexpr T&& constexpr_forward(typename std::remove_reference<T>::type& t) noexcept
{
	return static_cast<T&&>(t);
}

template <class T>
inline constexpr T&& constexpr_forward(typename std::remove_reference<T>::type&& t) noexcept
{
	static_assert(!std::is_lvalue_reference<T>::value, "!!");
	return static_cast<T&&>(t);
}

template <class T>
inline constexpr typename std::remove_reference<T>::type&& constexpr_move(T&& t) noexcept
{
	return static_cast<typename std::remove_reference<T>::type&&>(t);
}

#if defined NDEBUG
#define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) (EXPR)
#else
#define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) ((CHECK) ? (EXPR) : ([] { assert(!#CHECK); }(), (EXPR)))
#endif

namespace detail_
{

// static_addressof: a constexpr version of addressof
template <typename T>
struct has_overloaded_addressof
{
	template <class X>
	constexpr static bool has_overload(...)
	{
		return false;
	}

	template <class X, size_t S = sizeof(std::declval<X&>().operator&())>
	constexpr static bool has_overload(bool)
	{
		return true;
	}

	constexpr static bool value = has_overload<T>(true);
};

template <typename T, TR2_OPTIONAL_REQUIRES(!has_overloaded_addressof<T>)>
constexpr T* static_addressof(T& ref)
{
	return &ref;
}

template <typename T, TR2_OPTIONAL_REQUIRES(has_overloaded_addressof<T>)>
T* static_addressof(T& ref)
{
	return std::addressof(ref);
}

// the call to convert<A>(b) has return type A and converts b to type A iff b decltype(b) is implicitly
// convertible to A
template <class U>
constexpr U convert(U v)
{
	return v;
}

} // namespace detail

constexpr struct trivial_init_t
{
} trivial_init{};

// 20.5.6, In-place construction
#ifndef STX_HAVE_IN_PLACE_T

struct in_place_t
{
	explicit in_place_t() = default;
};

constexpr in_place_t in_place{};

// template <class T> struct in_place_type_t {
//    explicit in_place_type_t() = default;
//};

// template <size_t I> struct in_place_index_t {
//    explicit in_place_index_t() = default;
//};

//#if __cpp_variable_templates >= 201304
// template <class T>
// constexpr in_place_type_t<T> in_place_type{};
// template <size_t I>
// constexpr in_place_index_t<I> in_place_index{};
//#endif // __cpp_variable_templates

#define STX_HAVE_IN_PLACE_T
#endif // STX_HAVE_IN_PLACE_T

// 20.5.7, Disengaged state indicator
struct nullopt_t
{
	struct init
	{
	};
	constexpr explicit nullopt_t(init)
	{
	}
};
constexpr nullopt_t nullopt{nullopt_t::init()};

// 20.5.8, class bad_optional_access
class bad_optional_access : public std::logic_error
{
public:
	explicit bad_optional_access(const std::string& what_arg)
		: logic_error{what_arg}
	{
	}
	explicit bad_optional_access(const char* what_arg)
		: logic_error{what_arg}
	{
	}
};

template <class T>
union storage_t {
	unsigned char dummy_;
	T value_;

	constexpr storage_t(trivial_init_t) noexcept
		: dummy_(){}

	template <class... Args>
	constexpr storage_t(Args&&... args)
		: value_(constexpr_forward<Args>(args)...)
	{
	}

	~storage_t()
	{
	}
};

template <class T>
union constexpr_storage_t {
	unsigned char dummy_;
	T value_;

	constexpr constexpr_storage_t(trivial_init_t) noexcept
		: dummy_(){}

	template <class... Args>
	constexpr constexpr_storage_t(Args&&... args)
		: value_(constexpr_forward<Args>(args)...)
	{
	}

	~constexpr_storage_t() = default;
};

template <class T>
struct optional_base
{
	bool init_;
	storage_t<T> storage_;

	constexpr optional_base() noexcept
		: init_(false)
		, storage_(trivial_init){}

	explicit constexpr optional_base(const T& v)
		: init_(true)
		, storage_(v)
	{
	}

	explicit constexpr optional_base(T&& v)
		: init_(true)
		, storage_(constexpr_move(v))
	{
	}

	template <class... Args>
	explicit optional_base(in_place_t, Args&&... args)
		: init_(true)
		, storage_(constexpr_forward<Args>(args)...)
	{
	}

	template <class U, class... Args,
			  TR2_OPTIONAL_REQUIRES(std::is_constructible<T, std::initializer_list<U>>)>
	explicit optional_base(in_place_t, std::initializer_list<U> il, Args&&... args)
		: init_(true)
		, storage_(il, std::forward<Args>(args)...)
	{
	}

	~optional_base()
	{
		if(init_)
			storage_.value_.T::~T();
	}
};

template <class T>
struct constexpr_optional_base
{
	bool init_;
	constexpr_storage_t<T> storage_;

	constexpr constexpr_optional_base() noexcept
		: init_(false)
		, storage_(trivial_init){}

	explicit constexpr constexpr_optional_base(const T& v)
		: init_(true)
		, storage_(v)
	{
	}

	explicit constexpr constexpr_optional_base(T&& v)
		: init_(true)
		, storage_(constexpr_move(v))
	{
	}

	template <class... Args>
	explicit constexpr constexpr_optional_base(in_place_t, Args&&... args)
		: init_(true)
		, storage_(constexpr_forward<Args>(args)...)
	{
	}

	template <class U, class... Args,
			  TR2_OPTIONAL_REQUIRES(std::is_constructible<T, std::initializer_list<U>>)>
	OPTIONAL_CONSTEXPR_INIT_LIST explicit constexpr_optional_base(in_place_t, std::initializer_list<U> il,
																  Args&&... args)
		: init_(true)
		, storage_(il, std::forward<Args>(args)...)
	{
	}

	~constexpr_optional_base() = default;
};

template <class T>
using OptionalBase = typename std::conditional<
	std::is_trivially_destructible<T>::value,					  // if possible
	constexpr_optional_base<typename std::remove_const<T>::type>, // use base with trivial destructor
	optional_base<typename std::remove_const<T>::type>>::type;

template <class T>
class optional : private OptionalBase<T>
{
	static_assert(!std::is_same<typename std::decay<T>::type, nullopt_t>::value, "bad T");
	static_assert(!std::is_same<typename std::decay<T>::type, in_place_t>::value, "bad T");

	constexpr bool initialized() const noexcept
	{
		return OptionalBase<T>::init_;
	}
	typename std::remove_const<T>::type* dataptr()
	{
		return std::addressof(OptionalBase<T>::storage_.value_);
	}
	constexpr const T* dataptr() const
	{
		return detail_::static_addressof(OptionalBase<T>::storage_.value_);
	}

#if OPTIONAL_HAS_THIS_RVALUE_REFS == 1
	constexpr const T& contained_val() const &
	{
		return OptionalBase<T>::storage_.value_;
	}
#if OPTIONAL_HAS_MOVE_ACCESSORS == 1
	OPTIONAL_MUTABLE_CONSTEXPR T&& contained_val() &&
	{
		return std::move(OptionalBase<T>::storage_.value_);
	}
	OPTIONAL_MUTABLE_CONSTEXPR T& contained_val() &
	{
		return OptionalBase<T>::storage_.value_;
	}
#else
	T& contained_val() &
	{
		return OptionalBase<T>::storage_.value_;
	}
	T&& contained_val() &&
	{
		return std::move(OptionalBase<T>::storage_.value_);
	}
#endif
#else
	constexpr const T& contained_val() const
	{
		return OptionalBase<T>::storage_.value_;
	}
	T& contained_val()
	{
		return OptionalBase<T>::storage_.value_;
	}
#endif

	void clear() noexcept
	{
		if(initialized())
			dataptr()->T::~T();
		OptionalBase<T>::init_ = false;
	}

	template <class... Args>
	void initialize(Args&&... args) noexcept(noexcept(T(std::forward<Args>(args)...)))
	{
		assert(!OptionalBase<T>::init_);
		::new(static_cast<void*>(dataptr())) T(std::forward<Args>(args)...);
		OptionalBase<T>::init_ = true;
	}

	template <class U, class... Args>
	void initialize(std::initializer_list<U> il,
					Args&&... args) noexcept(noexcept(T(il, std::forward<Args>(args)...)))
	{
		assert(!OptionalBase<T>::init_);
		::new(static_cast<void*>(dataptr())) T(il, std::forward<Args>(args)...);
		OptionalBase<T>::init_ = true;
	}

public:
	typedef T value_type;

	// 20.5.5.1, constructors
	constexpr optional() noexcept
		: OptionalBase<T>(){}
	constexpr optional(nullopt_t) noexcept
		: OptionalBase<T>(){}

	optional(const optional& rhs)
		: OptionalBase<T>()
	{
		if(rhs.initialized())
		{
			::new(static_cast<void*>(dataptr())) T(*rhs);
			OptionalBase<T>::init_ = true;
		}
	}

	optional(optional&& rhs) noexcept(std::is_nothrow_move_constructible<T>::value)
		: OptionalBase<T>()
	{
		if(rhs.initialized())
		{
			::new(static_cast<void*>(dataptr())) T(std::move(*rhs));
			OptionalBase<T>::init_ = true;
		}
	}

	constexpr optional(const T& v)
		: OptionalBase<T>(v)
	{
	}

	constexpr optional(T&& v)
		: OptionalBase<T>(constexpr_move(v))
	{
	}

	template <class... Args>
	explicit constexpr optional(in_place_t, Args&&... args)
		: OptionalBase<T>(in_place_t{}, constexpr_forward<Args>(args)...)
	{
	}

	template <class U, class... Args,
			  TR2_OPTIONAL_REQUIRES(std::is_constructible<T, std::initializer_list<U>>)>
	OPTIONAL_CONSTEXPR_INIT_LIST explicit optional(in_place_t, std::initializer_list<U> il, Args&&... args)
		: OptionalBase<T>(in_place_t{}, il, constexpr_forward<Args>(args)...)
	{
	}

	// 20.5.4.2, Destructor
	~optional() = default;

	// 20.5.4.3, assignment
	optional& operator=(nullopt_t) noexcept
	{
		clear();
		return *this;
	}

	optional& operator=(const optional& rhs)
	{
		if(initialized() == true && rhs.initialized() == false)
			clear();
		else if(initialized() == false && rhs.initialized() == true)
			initialize(*rhs);
		else if(initialized() == true && rhs.initialized() == true)
			contained_val() = *rhs;
		return *this;
	}

	optional& operator=(optional&& rhs) noexcept(
		std::is_nothrow_move_assignable<T>::value&& std::is_nothrow_move_constructible<T>::value)
	{
		if(initialized() == true && rhs.initialized() == false)
			clear();
		else if(initialized() == false && rhs.initialized() == true)
			initialize(std::move(*rhs));
		else if(initialized() == true && rhs.initialized() == true)
			contained_val() = std::move(*rhs);
		return *this;
	}

	template <class U>
	auto operator=(U&& v) ->
		typename std::enable_if<std::is_same<typename std::decay<U>::type, T>::value, optional&>::type
	{
		if(initialized())
		{
			contained_val() = std::forward<U>(v);
		}
		else
		{
			initialize(std::forward<U>(v));
		}
		return *this;
	}

	template <class... Args>
	void emplace(Args&&... args)
	{
		clear();
		initialize(std::forward<Args>(args)...);
	}

	template <class U, class... Args>
	void emplace(std::initializer_list<U> il, Args&&... args)
	{
		clear();
		initialize<U, Args...>(il, std::forward<Args>(args)...);
	}

	// 20.5.4.4, Swap
	void swap(optional<T>& rhs) noexcept(
		std::is_nothrow_move_constructible<T>::value&& noexcept(swap(std::declval<T&>(), std::declval<T&>())))
	{
		if(initialized() == true && rhs.initialized() == false)
		{
			rhs.initialize(std::move(**this));
			clear();
		}
		else if(initialized() == false && rhs.initialized() == true)
		{
			initialize(std::move(*rhs));
			rhs.clear();
		}
		else if(initialized() == true && rhs.initialized() == true)
		{
			using std::swap;
			swap(**this, *rhs);
		}
	}

	// 20.5.4.5, Observers

	explicit constexpr operator bool() const noexcept
	{
		return initialized();
	}

	constexpr T const* operator->() const
	{
		return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), dataptr());
	}

#if OPTIONAL_HAS_MOVE_ACCESSORS == 1

	OPTIONAL_MUTABLE_CONSTEXPR T* operator->()
	{
		assert(initialized());
		return dataptr();
	}

	constexpr T const& operator*() const &
	{
		return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val());
	}

	OPTIONAL_MUTABLE_CONSTEXPR T& operator*() &
	{
		assert(initialized());
		return contained_val();
	}

	OPTIONAL_MUTABLE_CONSTEXPR T&& operator*() &&
	{
		assert(initialized());
		return constexpr_move(contained_val());
	}

	constexpr T const& value() const &
	{
		return initialized() ? contained_val()
							 : (throw bad_optional_access("bad optional access"), contained_val());
	}

	OPTIONAL_MUTABLE_CONSTEXPR T& value() &
	{
		return initialized() ? contained_val()
							 : (throw bad_optional_access("bad optional access"), contained_val());
	}

	OPTIONAL_MUTABLE_CONSTEXPR T&& value() &&
	{
		if(!initialized())
			throw bad_optional_access("bad optional access");
		return std::move(contained_val());
	}

#else

	T* operator->()
	{
		assert(initialized());
		return dataptr();
	}

	constexpr T const& operator*() const
	{
		return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val());
	}

	T& operator*()
	{
		assert(initialized());
		return contained_val();
	}

	constexpr bool has_value() const noexcept
	{
		return initialized();
	}

	constexpr T const& value() const
	{
		return initialized() ? contained_val()
							 : (throw bad_optional_access("bad optional access"), contained_val());
	}

	T& value()
	{
		return initialized() ? contained_val()
							 : (throw bad_optional_access("bad optional access"), contained_val());
	}

#endif

#if OPTIONAL_HAS_THIS_RVALUE_REFS == 1

	template <class V>
	constexpr T value_or(V&& v) const &
	{
		return *this ? **this : detail_::convert<T>(constexpr_forward<V>(v));
	}

#if OPTIONAL_HAS_MOVE_ACCESSORS == 1

	template <class V>
	OPTIONAL_MUTABLE_CONSTEXPR T value_or(V&& v) &&
	{
		return *this ? constexpr_move(const_cast<optional<T>&>(*this).contained_val())
					 : detail_::convert<T>(constexpr_forward<V>(v));
	}

#else

	template <class V>
	T value_or(V&& v) &&
	{
		return *this ? constexpr_move(const_cast<optional<T>&>(*this).contained_val())
					 : detail_::convert<T>(constexpr_forward<V>(v));
	}

#endif

#else

	template <class V>
	constexpr T value_or(V&& v) const
	{
		return *this ? **this : detail_::convert<T>(constexpr_forward<V>(v));
	}

#endif
};

template <class T>
class optional<T&>
{
	static_assert(!std::is_same<T, nullopt_t>::value, "bad T");
	static_assert(!std::is_same<T, in_place_t>::value, "bad T");
	T* ref;

public:
	// 20.5.5.1, construction/destruction
	constexpr optional() noexcept
		: ref(nullptr)
	{
	}

	constexpr optional(nullopt_t) noexcept
		: ref(nullptr)
	{
	}

	constexpr optional(T& v) noexcept
		: ref(detail_::static_addressof(v))
	{
	}

	optional(T&&) = delete;

	constexpr optional(const optional& rhs) noexcept
		: ref(rhs.ref)
	{
	}

	explicit constexpr optional(in_place_t, T& v) noexcept
		: ref(detail_::static_addressof(v))
	{
	}

	explicit optional(in_place_t, T&&) = delete;

	~optional() = default;

	// 20.5.5.2, mutation
	optional& operator=(nullopt_t) noexcept
	{
		ref = nullptr;
		return *this;
	}

	// optional& operator=(const optional& rhs) noexcept {
	// ref = rhs.ref;
	// return *this;
	// }

	// optional& operator=(optional&& rhs) noexcept {
	// ref = rhs.ref;
	// return *this;
	// }

	template <typename U>
	auto operator=(U&& rhs) noexcept ->
		typename std::enable_if<std::is_same<typename std::decay<U>::type, optional<T&>>::value,
								optional&>::type
	{
		ref = rhs.ref;
		return *this;
	}

	template <typename U>
	auto operator=(U&& rhs) noexcept ->
		typename std::enable_if<!std::is_same<typename std::decay<U>::type, optional<T&>>::value,
								optional&>::type = delete;

	void emplace(T& v) noexcept
	{
		ref = detail_::static_addressof(v);
	}

	void emplace(T&&) = delete;

	void swap(optional<T&>& rhs) noexcept
	{
		std::swap(ref, rhs.ref);
	}

	// 20.5.5.3, observers
	constexpr T* operator->() const
	{
		return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, ref);
	}

	constexpr T& operator*() const
	{
		return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, *ref);
	}

	constexpr T& value() const
	{
		return ref ? *ref : (throw bad_optional_access("bad optional access"), *ref);
	}

	explicit constexpr operator bool() const noexcept
	{
		return ref != nullptr;
	}

	template <class V>
	constexpr typename std::decay<T>::type value_or(V&& v) const
	{
		return *this ? **this : detail_::convert<typename std::decay<T>::type>(constexpr_forward<V>(v));
	}
};

template <class T>
class optional<T&&>
{
	static_assert(sizeof(T) == 0, "optional rvalue references disallowed");
};

// 20.5.8, Relational operators
template <class T>
constexpr bool operator==(const optional<T>& x, const optional<T>& y)
{
	return bool(x) != bool(y) ? false : bool(x) == false ? true : *x == *y;
}

template <class T>
constexpr bool operator!=(const optional<T>& x, const optional<T>& y)
{
	return !(x == y);
}

template <class T>
constexpr bool operator<(const optional<T>& x, const optional<T>& y)
{
	return (!y) ? false : (!x) ? true : *x < *y;
}

template <class T>
constexpr bool operator>(const optional<T>& x, const optional<T>& y)
{
	return (y < x);
}

template <class T>
constexpr bool operator<=(const optional<T>& x, const optional<T>& y)
{
	return !(y < x);
}

template <class T>
constexpr bool operator>=(const optional<T>& x, const optional<T>& y)
{
	return !(x < y);
}

// 20.5.9, Comparison with nullopt
template <class T>
constexpr bool operator==(const optional<T>& x, nullopt_t) noexcept
{
	return (!x);
}

template <class T>
constexpr bool operator==(nullopt_t, const optional<T>& x) noexcept
{
	return (!x);
}

template <class T>
constexpr bool operator!=(const optional<T>& x, nullopt_t) noexcept
{
	return bool(x);
}

template <class T>
constexpr bool operator!=(nullopt_t, const optional<T>& x) noexcept
{
	return bool(x);
}

template <class T>
constexpr bool operator<(const optional<T>&, nullopt_t) noexcept
{
	return false;
}

template <class T>
constexpr bool operator<(nullopt_t, const optional<T>& x) noexcept
{
	return bool(x);
}

template <class T>
constexpr bool operator<=(const optional<T>& x, nullopt_t) noexcept
{
	return (!x);
}

template <class T>
constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept
{
	return true;
}

template <class T>
constexpr bool operator>(const optional<T>& x, nullopt_t) noexcept
{
	return bool(x);
}

template <class T>
constexpr bool operator>(nullopt_t, const optional<T>&) noexcept
{
	return false;
}

template <class T>
constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept
{
	return true;
}

template <class T>
constexpr bool operator>=(nullopt_t, const optional<T>& x) noexcept
{
	return (!x);
}

// 20.5.10, Comparison with T
template <class T>
constexpr bool operator==(const optional<T>& x, const T& v)
{
	return bool(x) ? *x == v : false;
}

template <class T>
constexpr bool operator==(const T& v, const optional<T>& x)
{
	return bool(x) ? v == *x : false;
}

template <class T>
constexpr bool operator!=(const optional<T>& x, const T& v)
{
	return bool(x) ? *x != v : true;
}

template <class T>
constexpr bool operator!=(const T& v, const optional<T>& x)
{
	return bool(x) ? v != *x : true;
}

template <class T>
constexpr bool operator<(const optional<T>& x, const T& v)
{
	return bool(x) ? *x < v : true;
}

template <class T>
constexpr bool operator>(const T& v, const optional<T>& x)
{
	return bool(x) ? v > *x : true;
}

template <class T>
constexpr bool operator>(const optional<T>& x, const T& v)
{
	return bool(x) ? *x > v : false;
}

template <class T>
constexpr bool operator<(const T& v, const optional<T>& x)
{
	return bool(x) ? v < *x : false;
}

template <class T>
constexpr bool operator>=(const optional<T>& x, const T& v)
{
	return bool(x) ? *x >= v : false;
}

template <class T>
constexpr bool operator<=(const T& v, const optional<T>& x)
{
	return bool(x) ? v <= *x : false;
}

template <class T>
constexpr bool operator<=(const optional<T>& x, const T& v)
{
	return bool(x) ? *x <= v : true;
}

template <class T>
constexpr bool operator>=(const T& v, const optional<T>& x)
{
	return bool(x) ? v >= *x : true;
}

// Comparison of optional<T&> with T
template <class T>
constexpr bool operator==(const optional<T&>& x, const T& v)
{
	return bool(x) ? *x == v : false;
}

template <class T>
constexpr bool operator==(const T& v, const optional<T&>& x)
{
	return bool(x) ? v == *x : false;
}

template <class T>
constexpr bool operator!=(const optional<T&>& x, const T& v)
{
	return bool(x) ? *x != v : true;
}

template <class T>
constexpr bool operator!=(const T& v, const optional<T&>& x)
{
	return bool(x) ? v != *x : true;
}

template <class T>
constexpr bool operator<(const optional<T&>& x, const T& v)
{
	return bool(x) ? *x < v : true;
}

template <class T>
constexpr bool operator>(const T& v, const optional<T&>& x)
{
	return bool(x) ? v > *x : true;
}

template <class T>
constexpr bool operator>(const optional<T&>& x, const T& v)
{
	return bool(x) ? *x > v : false;
}

template <class T>
constexpr bool operator<(const T& v, const optional<T&>& x)
{
	return bool(x) ? v < *x : false;
}

template <class T>
constexpr bool operator>=(const optional<T&>& x, const T& v)
{
	return bool(x) ? *x >= v : false;
}

template <class T>
constexpr bool operator<=(const T& v, const optional<T&>& x)
{
	return bool(x) ? v <= *x : false;
}

template <class T>
constexpr bool operator<=(const optional<T&>& x, const T& v)
{
	return bool(x) ? *x <= v : true;
}

template <class T>
constexpr bool operator>=(const T& v, const optional<T&>& x)
{
	return bool(x) ? v >= *x : true;
}

// Comparison of optional<T const&> with T
template <class T>
constexpr bool operator==(const optional<const T&>& x, const T& v)
{
	return bool(x) ? *x == v : false;
}

template <class T>
constexpr bool operator==(const T& v, const optional<const T&>& x)
{
	return bool(x) ? v == *x : false;
}

template <class T>
constexpr bool operator!=(const optional<const T&>& x, const T& v)
{
	return bool(x) ? *x != v : true;
}

template <class T>
constexpr bool operator!=(const T& v, const optional<const T&>& x)
{
	return bool(x) ? v != *x : true;
}

template <class T>
constexpr bool operator<(const optional<const T&>& x, const T& v)
{
	return bool(x) ? *x < v : true;
}

template <class T>
constexpr bool operator>(const T& v, const optional<const T&>& x)
{
	return bool(x) ? v > *x : true;
}

template <class T>
constexpr bool operator>(const optional<const T&>& x, const T& v)
{
	return bool(x) ? *x > v : false;
}

template <class T>
constexpr bool operator<(const T& v, const optional<const T&>& x)
{
	return bool(x) ? v < *x : false;
}

template <class T>
constexpr bool operator>=(const optional<const T&>& x, const T& v)
{
	return bool(x) ? *x >= v : false;
}

template <class T>
constexpr bool operator<=(const T& v, const optional<const T&>& x)
{
	return bool(x) ? v <= *x : false;
}

template <class T>
constexpr bool operator<=(const optional<const T&>& x, const T& v)
{
	return bool(x) ? *x <= v : true;
}

template <class T>
constexpr bool operator>=(const T& v, const optional<const T&>& x)
{
	return bool(x) ? v >= *x : true;
}

// 20.5.12, Specialized algorithms
template <class T>
void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y)))
{
	x.swap(y);
}

template <class T>
constexpr optional<typename std::decay<T>::type> make_optional(T&& v)
{
	return optional<typename std::decay<T>::type>(constexpr_forward<T>(v));
}

template <class X>
constexpr optional<X&> make_optional(std::reference_wrapper<X> v)
{
	return optional<X&>(v.get());
}

} // namespace

namespace std
{
template <typename T>
struct hash<STX_NAMESPACE_NAME::optional<T>>
{
	using result_type = typename hash<T>::result_type;
	using argument_type = nonstd::optional<T>;

	constexpr result_type operator()(argument_type const& arg) const
	{
		return arg ? std::hash<T>{}(*arg) : result_type{};
	}
};

template <typename T>
struct hash<STX_NAMESPACE_NAME::optional<T&>>
{
	typedef typename hash<T>::result_type result_type;
	typedef STX_NAMESPACE_NAME::optional<T&> argument_type;

	constexpr result_type operator()(argument_type const& arg) const
	{
		return arg ? std::hash<T>{}(*arg) : result_type{};
	}
};
}

#undef TR2_OPTIONAL_REQUIRES
#undef TR2_OPTIONAL_ASSERTED_EXPRESSION

#endif // STX_NO_STD_OPTIONAL

#endif // STX_OPTIONAL_HPP_INCLUDED
